When a compressor is used to compress a refrigerant, such as “Freon”, charged in a refrigerating circuit of an air conditioning system, it has been customary to use a lip-type high pressure seal in order to seal a shaft of the compressor.
The conventional lip-type high pressure seal is comprised of a first sealing lip made of a low friction material such as “Teflon” (polytetrafluoroethylene. PTFE) and a second sealing lip made of a resilient elastomeric material such as nitrite rubber or fluorine rubber, as described, for example, in JP-U-2-146269, JP-U-2-110760, JP-U-3-102658 and JP-A-11-125337.
The second sealing lip made of the resilient elastomeric material primarily serves to provide a static seal when the shaft is not rotating.
PTFE which forms the first sealing lip is relatively self-sustaining or stiff as compared with the elastomeric material forming the second sealing lip and has an excellent wear resistivity and heat resistivity, so that the first sealing lip is able to well withstand a high pressure refrigerant.
Furthermore, the first sealing lip made of PTFE is provided, on the inner circumferential face thereof in contact with the compressor shaft, with helical pumping elements adapted to hydrodynamically pump a fluid such as a lubricant, that has leaked from the sealed side to the atmospheric side, back to the sealed side, so that the first sealing lip functions to provide a dynamic seal when the shaft is rotating.
In order to avoid destruction of the ozone layer by “Freon” and to thereby preserve the global environment, proposed today in the art is the use of hydrocarbons, ammonia and carbon dioxide which may be used as a refrigerant in place of “Freon”. Among these, carbon dioxide is considered preferable from the view point of environmental contamination and safety as it is harmless.
However, the problem which must be overcome in realizing a refrigerating system wherein carbon dioxide gas is used as a refrigerant is that the shaft seals of the compressor are subjected to an extremely high pressure which has never been experienced before in the field of fluid seals. More specifically, referring to the Moldier diagram shown in FIG. 1, in contrast to the conventional refrigerating circuit wherein “Freon 134a” is used as the refrigerant so that the refrigerant pressure remains in the range of about 0.4–1.4 MPa, it is anticipated that, in a refrigerating system wherein carbon dioxide is used, the pressure of carbon dioxide gas entering the compressor will be as extremely high as about 4–12 MPa.
In addition to the fact that the shaft seals of the compressor are subjected to carbon dioxide at extremely high gas pressure of about 4–12 MPa as aforementioned, the carbon dioxide gas inherently has a good affinity to a polymer material and is, therefore, highly permeable to the polymer material. As a result, carbon dioxide gas charged in a refrigerating circuit would be prematurely lost if the conventional lip seals made of PTFE and rubber are used. In this regard, according to the testing and experiments carried out by the present inventors, it has been found that the conventional material such as PTFE and rubber which has been used to form the conventional lip seals exhibits a high permeability against carbon dioxide gas so that the amount of carbon dioxide gas leakage which would occur under a high gas pressure of more than about 4 MPa would be prohibitive in realizing a commercially feasible refrigerating circuit.
The second problem which the shaft seals of the compressor must overcome in realizing the refrigerating system wherein carbon dioxide gas is used as a refrigerant is that, in the course of the refrigerating cycle of the refrigerating machine, carbon dioxide gas is caused to pass a supercritical state. As shown in the phase diagram of FIG. 2, carbon dioxide gas becomes a supercritical gas at a temperature higher than the critical temperature of 31.06° C. and at a pressure higher than the critical pressure of 7.38 MPa. As supercritical carbon dioxide gas exhibits a density which is closer to that of a liquid than a gas, it is soluble in a large amount into the PTFE material forming the lip-type seal. Carbon dioxide gas having been dissolved into the material will evaporate when the pressure of the refrigerating circuit is lowered, thereby causing foaming of the PTFE material. As a result, PTFE forming the lip-type seals will be degraded and destroyed by repeated dissolution and foaming of the supercritical carbon dioxide gas so that the mechanical strength of the lip-type seals will be prematurely lost.
Another problem of the refrigerating circuit using carbon dioxide gas is that, due to the high pressure of carbon dioxide gas which pressure is as high as about 4–12 MPa, the lip-type seals will be locally subjected to a tensile effort which would surpass the limit of elasticity of PTFE. As PTFE has a relatively low modulus of elasticity, the sealing lip made of PTFE will undergo local plastic deformation and, hence, permanent deformation to thereby result in a loss or degradation of the static sealing capability if a tensile stress surpassing the elasticity limit of PTFE is exerted.
Accordingly, an object of the present invention is to provide a high pressure shaft seal which is suitable for sealing a gas under an extremely high pressure and which can be used in a refrigerating system wherein carbon dioxide gas is used as a refrigerant.
Another object of the invention is to provide a high pressure shaft seal which is suitable for sealing a highly permeable gas such as supercritical carbon dioxide gas.
A still further object of the invention is to provide a method for establishing fluid seal between a compressor shaft and a housing against an extremely high fluid pressure which may be encountered in a refrigerating system wherein carbon dioxide gas is used as a refrigerant.
A further object of the invention is to provide a shaft seal which is able to exhibit a good sealing capability under a high fluid pressure which may reach as high as about 12 MPa.